Temperature distribution downhole can be part of the data that a well operator needs to monitor downhole conditions. One way this information has been obtained in the past is through a fiber optic cable that extends from the surface to the downhole completion(s) and gives data at the surface of the sensed temperature at any point along the fiber optic cable. The problem is that to accommodate the various equipment on the string as well as to facilitate assembly of the string and associated equipment, requires that slack be built into the fiber optic cable. Generally, this slack is provided by adding coils around portions of the string. The slack that is provided allows running in with minimal damage to the cable and facilitates assembly of the string and associated equipment that it supports.
The problem is that the provision of slack at one or multiple locations along the length of the cable creates a disassociation between the position along the length of the cable and the physical location of that portion of the cable with respect to the running length of tubular into the well. As a result, it becomes unclear as to where in the well the temperature profile transmitted through the cable is actually located in the well.
Additionally, an optical fiber cable within a line can have a variable length, which can occur as a result in variability of the overstuffing used when installing the fiber optic cable into the line. Optical fiber may be inserted into the line during either manufacture of the line prior to downhole installation, or after the line has been installed downhole. Overstuffing may occur as a natural consequence of the manufacturing process, but is also done intentionally to compensate for differential rates of thermal expansion between the cable itself and the line into which it is placed. Typically the overstuffing can account for a few tenths of a percent of the overall length but can vary from about 1% to several percent of the cable length.
Another uncertainty in depth correlation of the readings obtained through a fiber optic is the variability of the refractive index of the fiber optic material in bulk or as a function of location along its length. The refractive index determines the speed at which light travels in the optical fiber cable, therefore for fiber optic measurement techniques such as optical time-domain reflectometry (OTDR) and other intrinsic sensing techniques that rely on knowledge of the optical fiber refractive index, errors in estimating the refractive index of the optical fiber creates errors in positional accuracy of the measurement. The present invention allows the use of location markers at known depths to correlate the received data to a depth while minimizing the uncertainties from the variables discussed above.
While the context of the invention is described in terms of a fiber optic measuring temperature, the scope of the invention includes other systems where there is not a direct correlation, for whatever reason, between line length and string length. It should be noted that another reason slack is deliberately added to a line supported by a tubing string is that well conditions or supported weight can result in length changes of the string itself and the slack in the associated cable that it supports is put there to allow the cable to grow with the string that supports it without damage such as a tensile stress that can result in the shear failure of the cable.
The present invention addresses the need to correlate a specific length along the cable with a location along the supporting tubular downhole. It does this by placing a heat source at a known location on the string and sensing its output at a known location on the cable. In fact the correlation signal can be any signal that can be transmitted through the cable such as a vibration signal, as one example. From one or more correlation locations the results seen at the surface from the cable can be correlated to a physical location in the wellbore. While the preferred embodiment will be described in detail below in the context of correlation using temperature as the variable, those skilled in the art will understand that the invention relates to correlation techniques in general regardless of the measured variable. The correlation can also be provided in real time or periodically on a sample interval basis. These and other aspects of the present invention will be more apparent to those skilled in the art from a review of the description of the preferred embodiment and the associated drawings while the full scope of the invention will be found in the claims attached below.